Gaining weight? Here's why you should look to your friends

Are your friends sporty and healthy eaters, or more of a beer and pizza crowd? A new study out of the US finds that the answer could provide some insight into the current state of your body.

To conduct the study, David Shoham and his team at Loyola University Chicago Stritch School of Medicine examined data from two large suburban high schools that participated in the National Longitudinal Study of Adolescent Health in 1994 and 1995.

Over the course of two years, the students were surveyed about their weight, friendships, sports activities and television and computer time. Researchers also calculated each student's body mass index (BMI).

The results, published in the journal PLoS ONE last month, found that students were more likely to gain weight if they had friends who were heavier than they were. The converse was also true: students were more likely to lose weight -- or gain weight at a slower pace -- if they had a slimmer groups of friends.

Findings suggest that your friends may influence your lifestyle decisions both directly and indirectly, according to the researchers. Directly, your friends may nudge you toward say, going for a bike ride or trying a new yoga class, or conversely, sharing a triple fudge sundae. Indirectly, your friends could influence what you regard as appropriate body size and eating and exercise habits.

A separate study published in the same journal in February finds that women tend to mimic the eating behaviors of their friends, bite for bite, when dining out.

A 2007 study published in The New England Journal of Medicine also found that both obesity and thinness were socially contagious and influenced the social network's body weight: if one person is obese, odds that his or her friends will also become obese increases by 50 percent, the study found.